Umklapp scattering

what causes this? why is it important? I don't understnad why the phonon momentum is different but the k-vectors outside the first Brillouin zone are physically equivalent to vectors inside it. I can see that physically the vector is shifted by pi back into the Brillouin zone by G. Is the k vector representing momentum of the phonon? if not, what is?
n process on the left, u process on the right